If value in one input is Zero, return value from a second input.

[Sarge]

I am new to Alteryx and could use some help I haven’t found by searching the community, although I am sure there is info out there.  I have two data sources.  One has inventory data and the other has sales.  I need to return sales units from one source when the inventory units are zero in the other.  Otherwise I want the inventory units.  I did a union and that works when there is no value in the inventory file but it doesn’t work when there is a zero.  I assume there is some sort of if then statement but could use some guidance.

[Qiu]

@Sarge 
If you could provide some sample data, I am sure someone will help.